Federal regulators have discovered fragments of bird flu virus in roughly 20 percent of retail milk samples tested in a nationally representative study, the Food and Drug Administration said in an online update on Thursday.

Samples from parts of the country that are known to have dairy herds infected with the virus were more likely to test positive, the agency said. Regulators said that there is no evidence that this milk poses a danger to consumers or that live virus is present in the milk on store shelves, an assessment public health experts have agreed with.

But finding traces of the virus in such a high share of samples from around the country is the strongest signal yet that the bird flu outbreak in dairy cows is more extensive than the official tally of 33 infected herds across eight states.

“It suggests that there is a whole lot of this virus out there,” said Richard Webby, a virologist and influenza expert at St. Jude Children’s Research Hospital.

Dr. Webby said that he believed it was still possible to eradicate the virus, which is known as H5N1, from the nation’s dairy farms. But it will be difficult to design effective control measures without knowing the scope of the outbreak, he said.

The findings also raise questions about how the virus has evaded detection and where else it might be silently spreading. Some scientists have criticized the federal testing strategy as too limited to reveal the true extent of viral spread.

Until Wednesday, when the Department of Agriculture announced mandatory testing of dairy cows moving across state lines, testing of cows had been voluntary and primarily focused on cows with obvious symptoms.

As of Wednesday, just 23 people had been tested for the virus, while 44 people were being monitored after exposure to it, according to the Centers for Disease Control and Prevention.

A widespread outbreak in cows would pose a greater risk to farm workers, the dairy industry and public health more broadly. Sustained spread among cows would give the virus more opportunities to acquire mutations that make it more transmissible among humans.

The F.D.A. did not provide details on Thursday regarding the number or sources of the samples.

“You’d want to go not just to the places you knew there was activity and cows — you want to go to places where at least there’s no reported” bird flu, Dr. Webby said.

Experts believe that the process of pasteurization, in which milk is briefly heated, should inactivate this bird flu virus, which known as H5N1.

“And when you destroy the virus, it’s going to release genetic material,” said Samuel Alcaine, a microbiologist and food scientist at Cornell University. The genetic fragments left behind are not capable of causing infection.

“It’s not surprising” to find them in milk, he added. “It doesn’t mean that the milk is not safe.”

Federal officials are still conducting the time-intensive tests required to determine whether any viable virus remains in the milk after pasteurization. Scientists have said that prospect is very unlikely.

Dr. Jeanne Marrazzo, the director of the National Institute of Allergy and Infectious Diseases, said at a news briefing Wednesday that some federally sponsored researchers had tested for live virus in retail milk but had not found any, a sign that pasteurization had killed the virus before the milk reached grocery shelves.

Dr. Marrazzo cautioned that while the results were a small sample, the findings were “welcome news.”

“To really understand the scope here, we need to wait for the F.D.A. efforts,” she said.

Finding traces of the virus in 20 percent of commercial milk samples does not mean that 20 percent of the nation’s dairy herds are infected, experts cautioned. “It’s too early to try to do that back-of-the-napkin kind of calculation,” Dr. Alcaine said.

Milk from several farms is typically pooled. If the virus turns up in lots of milk samples drawn from one pool, it could mean that many cows are infected — or that a smaller number of infected cows are shedding large quantities of virus, Dr. Alcaine said.

Even in the latter case, however, a 20 percent positivity rate would suggest far more than 33 herds are infected, he noted.

In the Wednesday news briefing, Dr. Donald A. Prater, the acting director of the F.D.A.’s Center for Food Safety and Applied Nutrition, pointed to the novelty of the research effort. No studies have ever been completed on the effects of pasteurization on the bird flu virus in milk, he said.

Regulators were examining milk at various points in the commercial supply chain, he added, including milk on grocery shelves, as well as studying potential differences among milk products, such as those between whole milk and cream, Dr. Prater said.